Prolidase is a cytosolic exopeptidase that cleaves iminodipeptides with carboxy-terminal proline or hydroxyproline and plays major role in collagen turnover. Collagen is the essential content in atherosclerotic plaque playing a key role in the stability/instability of and progression of coronary artery disease (CAD). Consequently, in this study we sought to determine serum prolidase activity and markers of oxidative stress such as lipid hydroperoxide and total free sulfhydryl in CAD.We have evaluated 199 patients with CAD and 122 control cases with clinical, electrocardiographic, and laboratory investigation. We have measured serum prolidase activity and serum total free sulfhydryl levels spectrophotometrically. Serum lipid hydroperoxide levels were determined with ferrous ion oxidation-xylenol orange method. We assessed the association of serum prolidase activity with the presence and severity of CAD and clinical characteristics, and laboratory parameters.Serum prolidase activity (52.5+/-5.6 vs. 46.7+/-5.1 U/l, respectively, P<0.001) and serum lipid hydroperoxide levels were significantly increased in patients with CAD compared with control cases whereas, serum total free sulfhydryl levels were significantly decreased in patients with CAD compared with control cases. Serum prolidase activity and total free sulfhydryl levels were independent predictors of the presence of CAD [(chi=75.532, ss=0.212, P=0.003) and (chi=25.969, ss=-30.486, P=0.019), respectively] and Gensini score [(beta=0.276, P<0.001) and (beta=-0.274, P<0.001), respectively]. Independent predictors of serum prolidase activity were serum high-density lipoprotein cholesterol (beta=-0.138, P=0.023) and urea levels (beta=0.146, P=0.036), and Gensini score (beta=0.317, P<0.001).Findings of this study have shown that serum prolidase activity is significantly associated with the presence and severity of CAD, and elevated serum prolidase activity might be an independent predictor of coronary atherosclerosis.